The present invention relates to a seat belt retractor.
A seat belt retractor generally comprises a cylindrical bobbin or spool with a circular cross-section. Seat belt webbing is attached to and wound around the spool and the spool is mounted on a spool shaft in the seat belt retractor. The spool is rotatable to wind in webbing under action of a retractor spring and to pay out webbing under the influence of relatively gentle forwardly directed movement of a vehicle occupant, for example to allow for normal movement associated with vehicle occupancy such as reaching forwards to activate controls for a radio or a window, or to reach a glove compartment or door pocket. In a crash, the more extreme momentum of the vehicle occupant activates a crash sensor which locks the spool against rotation and thus prevents forward motion of the vehicle occupant.
The sudden locking of the seat belt spool in a crash can itself sometimes cause injury to the vehicle occupant due to sudden impact of the torso with the belt webbing. This is particularly true in severe crashes. In recent years this problem has been recognized and some solutions proposed.
One solution has been to rely on natural elongation of the webbing under high loads, and on the spool film effect caused by natural tightening of the webbing wound on the spool under high loads, to produce a load limiting effect.
Another proposal has been disclosed in U.S. Pat. No. 5,014,927, wherein force limiting is effected by interposing a plastically or electrically deformable member in the force path, for example between the spool and the innermost winding of the belt webbing. One common example is a torsion bar made of steel which twists when high torque is applied and which can rotate up to seven or eight times while remaining intact. The pay-out of webbing is generally in proportion to the momentum of the vehicle occupant at the moment a crash is sensed. This pay-out lessens the otherwise severe restraining forces on the vehicle occupant at high vehicle speeds, especially during the initial moments of the crash. It thus reduces the injurious effects of the seat belt in a crash.
However there is currently no straightforward way to adjust a torsion bar load limited seat belt retractor for different vehicle occupants. The weight and size of a vehicle occupant and the severity of the crash affects the performance of torsion bar load limiters and it would be preferable to adjust the load limiting to suit the vehicle occupant more closely.
There is provided in accordance with the present invention a seat belt retractor comprising: a spool mounted for rotation on a retractor frame for retraction or pay-out of seat belt webbing depending upon the direction of rotation of the spool; a means for locking the spool against rotation when a crash is sensed; a force limiting means for allowing further pay-out of the seat belt webbing after the spool shaft has locked, under the influence of a vehicle occupant""s forward momentum, wherein the force limiting means comprises: (a) a torsion bar attached in the force path between the spool and its axis of rotation; (b) a resilient mechanism, and means for selectively connecting the resilient mechanism into the force path between the spool and its axis of rotation; and (c) a means for selectively activating the connecting means under control of a control signal.